This invention relates to multiple photoflash lamp units, such as flashcubes and planar arrays of the type which are adapted to be temporarily attached to a camera.
It is not always possible to determine from the appearance of the lamps whether they have been fired. Therefore, when multiple flash lamp units are used, the photographer must remember how many lamps of the unit have been fired. If the operator initiates a film exposure requiring flash illumination when there is no unfired lamp available, the film exposure is wasted.
U.S. Pat. Nos. 3,894,226 to Hanson and 3,952,320 to Blount disclose a multiple flash lamp unit having two arrays each of four lamps. This unit can be connected to a camera in two orientations, in each of which one array of the flash lamps is farther from the camera lens axis than are the lamps of the other array. The lamps and electrical circuitry are arranged so that in each orientation of the unit with respect to the camera, only the group of lamps farther from the lens axis can be flashed. The purpose of this arrangement is to position the "active" group of flash lamps further above the camera lens, in order to reduce the possibility of a "red-eye" effect, in which the pupils of a person's eyes appear red or pink in pictures taken by flash when the flash lamp is close to the camera lens.
When flash lamp units of this type are used, the problem of remembering how many lamps have been fired is more severe, since the operator must keep track of how many lamps have been fired in a particular orientation of the unit. If the operator removes the unit to reverse its position, it may easily be incorrectly replaced with no unfired lamp available.
Since lamps of the type employed in the flash unit described herein present a very high resistance both before and after firing, it has not been practical to employ simple electric circuitry in the camera to detect the resistance condition of the lamps in the flash unit. Circuitry capable of distinguishing the fired from unfired condition would greatly increase the cost of the camera with which the flash unit is employed.
Some multiple flash lamp units have provided mechanically, thermally, or electrically actuated visible signals, intended to warn the operator when all lamps have been fired. Such designs have not been wholly satisfactory, because the signal may be overlooked and the film exposed in spite of the signal.
It is therefore an object of this invention to provide a multiple photoflash lamp unit that includes means for indicating that no unfired lamp is available.
Another object is to provide a lamp unit including multiple lamps whose resistance is not substantially altered by firing, having means for automatically providing an electrically detectable signal at completing of lamp firing.
It is a further object of this invention to provide a dual array photoflash unit, reversible between two orientations with respect to the camera, with means for indicating that no unfired lamp is available in a given array orientation.
To achieve these objects, a photoflash unit is provided for use with a camera having selectively operative means for providing film exposure, a firing pulse source, and inhibiting means coupled to the film exposure means and including resistance condition detecting means for sensing first and second resistance conditions of said photoflash unit and in response to the second resistance condition for inhibiting operation of the film exposure means.
The photoflash unit comprises an array of photoflash lamps adapted to be connected to said firing pulse source and arranged to be fired individually and sequentially thereby, and including a last lamp arranged to be fired last. According to the invention, the photoflash unit further comprises radiant energy responsive switching means, disposed to receive radiant energy emitted by the last lamp, to initially provide the first resistance condition and, responsive to firing of the last lamp, to provide the second resistance condition.
In preferred embodiments, the photoflash unit further provides first and second electric terminals adapted to be connected to the camera firing pulse source, the photoflash lamps being connected between the first and second terminals. The unit also provides a third terminal adapted to be connected to the camera resistance condition detecting means, and the radiant energy responsive switching means is connected between the third terminal and one of the other two. The switching means initially provides an open circuit or high resistance condition, and provides a low resistance or short circuit condition responsive to firing of the last lamp of the array.